What is Anhydrite?
Anhydrite is an evaporite mineral that occurs in extensive layered deposits in sedimentary basins where large volumes
of sea water have been evaporated. It is typically interbedded with halite, gypsum and limestone in accumulations
that can be up to hundreds of feet thick. On a much smaller scale, anhydrite can form in shoreline or tidal flat
sediments from the evaporation of sea water.
Anhydrite also occurs as a vein-filling mineral in hydrothermal deposits. It is deposited from solution, often along
with calcite and halite, as gangue in sulfide mineral deposits. Anhydrite is also found in the cap rock of salt
domes and in cavities of trap rock.
Anhydrite is an anhydrous calcium sulfate with a composition of CaSO4. It is closely related
to gypsum which has a chemical composition of CaS04.2H2O.
The worldwide abundance of gypsum greatly exceeds the abundance of anhydrite.
Anhydrite receives its name from
the Greek "anhydrous" which means "without water". It readily converts to gypsum under humid conditions or in contact with
ground water. This transition involves the absorption of water and a significant change in volume. That expansion can cause
deformation in the rock units. If gypsum is heated to about 200 degrees Celsius it will yield water and be converted to anhydrite.
This reaction occurs much less often.
Physical Properties and Identification
One of the most distinctive properties of anhydrite is its cubic cleavage. It cleaves in three directions at right angles. This can
easily be seen in coarsely crystalline specimens or with a hand lens in fine-grained specimens. This distinctive cleavage has earned
anhydrite the nickname of "cube spar".
Anhydrite can be a small challenge to identify when it occurs in massive form. It can be confused with gypsum, calcite or halite - which
it is almost always associated with. Compared to gypsum anhydrite exhibits cleavage in three directions at right angles and has a
greater hardness. Its right angle cleavage and lack of acid reaction allows it to be distinguished from calcite. Compared to halite it
is insoluble and slightly harder.
Physical Properties of Anhydrite |
| Color |
colorless, white, and light shades of brown, red, gray, pink, blue, violet |
| Streak |
White |
| Luster |
Vitreous to pearly |
| Diaphaneity |
Transparent to translucent |
| Cleavage |
Perfect cleavage in three directions to form cubic-shaped cleavage fragments |
| Mohs Hardness |
3 to 3.5 |
| Specific Gravity |
2.9 to 3.0 |
| Distinguishing Characteristics |
cubic cleavage, harder than gypsum, higher specific gravity than calcite, no acid reaction |
| Chemical Composition |
CaSO4 |
| Crystal System |
Orthorhombic |
Uses of Anhydrite
Anhydrite can be substituted for gypsum in some of its uses. Both minerals are crushed for use as a soil treatment
and in this purpose anhydrite is superior. One ton of anhydrite has more calcium than one ton of gypsum - because gypsum is about 21% water by weight. This yields more calcium per ton in a soil application. Anhydrite also
has a higher solubility which helps it benefit the soil quickly.
Small amounts of anhydrite are used as drying agents in plaster, paint and varnish. It is also used along with
gypsum to produce plaster, joint compound, wallboard and other products for the construction industry. Anhydrite
has also been used as a source of sulfur in the production of sulfuric acid.
Synthetic Anhydrite
Hydrofluoric acid is produced using fluorite and sulfuric acid. For every ton of hydrofluoric acid produced
about 3 1/2 tons of synthetic anhydrite is produced. For decades this synthetic anhydrite was considered to be a nuisance product
that had a disposal expense. However, much of it is now dried in a kiln and used as a raw material for
producing cement, plaster and flooring. It is also used as a filler in the production of plastics and paper
products.
Contributor: Hobart King
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| Anhydrite from Balmat, New York. This specimen of massive anhydrive has a typical gray color and a sugary appearance on broken surfaces caused by exposure of cleavage faces.
Specimen is approximately 4 inches (10 centimeters) across. |
| Massive anhydrite from Balmat, New York showing sedimentary layering and sugary appearance on broken surfaces.
Specimen is approximately 4 inches (10 centimeters) across. |
| Anhydrite from Mound House, Nevada with a very fine texture that could be confused with lithographic limestone.
Specimen is approximately 4 inches (10 centimeters) across. |
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